MOSFET's (metal-oxide-semiconductor field-effect transistor) or IGBT's (insulated gate bipolar transistor), which are broadly used as semiconductor devices for power, have a high-speed switching characteristic and a backward stopping voltage (withstand voltage) of tens to hundreds of V and are widely used in power conversion and control in electric home appliances, communication appliances, motors for vehicle mounting and the like. In order to achieve miniaturization, high efficiency, and reduced power consumption of these appliances, it is necessary to maintain high-voltage while lowering on-state resistance of the semiconductor devices.